In this study the DNA of selected human herpesviruses will be characterized, and the DNA base sequence homologies among those herpesviruses investigated. Of particular interest are the oral and venereal strains of herpes simplex virus and isolates of varicella-zoster virus. For herpes simplex virus, interest is concentrated on the repeated sequences, which are to be cloned. The method for the study of base sequence homology is to construct a DNA heteroduplex composed, for example, of the one cloned DNA strand from herpes simplex virus type 1 and the complementary cloned DNA strand from herpes simplex virus type 2. The heteroduplex is mounted for observation in the electron microscope, along with appropriate length standards. Regions of sufficient homology will appear duplex. Regions of insufficient homology will exist as two unpaired single strands. Detailed physical maps of DNA base sequence homology can be achieved. For varicella-zoster virus DNAs, comparisons of restriction enzyme cleavage patterns of the DNAs from fresh clinical isolates are easier for technical reasons than heteroduplex analysis. In particular, the DNA of a live, attenuated vaccine strain of varicella-zoster virus will be carefully compared to the DNA of the parental wild-type isolate. These related studies are of immediate relevance to human health, because herpes simplex virus causes widespread oral and venereal infections in the human population and varicella-zoster virus causes chicken pox, a highly contagious, ubiquitous disease of childhood.